ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Sunday April 7th, 2019
Posted by Aleph Johnston-Bloom on 04/07/19 at 7:00 am.
Avalanche risk
The Bottom Line
Moderate Avalanche Danger
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

The avalanche danger is  MODERATE  today at all elevations. Triggering a shallow wind slab will be possible on steep leeward features in the Alpine due to new snow and wind.  Be prepared for small natural wet loose activity on solar aspects in the Alpine if the sun makes an appearance today.  Below 3000′ glide cracks may avalanche without warning in popular terrain. Avoid traveling under glide cracks and give cornices a wide berth.  

PORTAGE VALLEY:  Summer trails with avalanche terrain overhead, such as Byron Glacier Trail, are not recommended due to the possibility of an avalanche or cornice fall sending debris over the trail.

2. Moderate
Alpine
/ Above 2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Treeline
/ 1,000'-2,500'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
2. Moderate
Below Treeline
/ Below 1,000'
Avalanche risk
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Special Announcements

Avalanche Problem 1
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
  • TYPE
    Wind Slabs
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Yesterday’s storm was mostly blow and not much snow… Rain/snow showers across the region deposited a few inches of new snow (3-6″/0.2-0.6 SWE) above approximately 2000′ favoring Girdwood over Turnagain Pass. Portage was the exception with the Bear Valley RWIS weather station recording 1.8″ of SWE in the past 24 hrs.  In the afternoon easterly winds ramped up blowing 15-25 mph with gusts into the 40s. The new snow fell onto a variety of surfaces, including firm crusts on W-S-E facing aspects in the Alpine and surface hoar and near surface facets on shaded northern aspects. Today steep leeward terrain features in the Alpine will be the most suspect for triggering a shallow wind slab. Watch for cracking and look for pillowed snow along ridgelines. Be cautious of fresh drifts. 

WET LOOSE: Keep in mind its springtime and be ready for small wet loose activity on solar aspects in the Alpine if the sun comes out. In the lower elevations below 2000’ where a stout crust has formed pay attention to supportability. If this crust starts to deteriorate and becomes punchy, triggering a wet loose avalanche in steep terrain will be possible.

CORNICES: Cornices remain very large along some ridgelines in the Alpine. Give them a wide berth and avoid travel directly below them.

South of Turnagain in Summit Lake and areas in the interior Kenai Peninsula have a variety of old weak layers within the snowpack. Triggering a persistent slab 2-3’ deep is becoming an outlier at this time, but the poor snowpack structure is worth noting if headed to Summit Lake. A stout crust has formed on all elevations below 3000’ and on solar aspects in the Alpine.  Be aware of a variety of avalanche conditions ranging from small isolated wind slabs to wet loose.  

Note the bump in the winds yesterday afternoon, 4-6-19

 What’s up with the crust today? Is the new snow bonding to the crust at upper elevations? Did the rain make the crust punchy down low?

Avalanche Problem 2
Glide Avalanches
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
  • TYPE
    Glide Avalanches
  • Chance
    Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size
    Historic
    Very Large
    Large
    Small

Glide cracks have been opening and releasing across our region over the last two weeks. This is the entire snowpack gliding downhill and spontaneously releasing. So far all of this activity has occurred on East, South, and West facing slopes below 3000’. This past week Turnagain Pass has been the most active. Glide avalanches are not associated with a human trigger and release without warning. The best way to manage this problem is avoidance. Identify existing cracks and plan your route to avoid being under a glide. Many cracks are opening in popular terrain and may require an unusual route to avoid their runout. This is a case of ‘don’t be caught in the wrong place at the wrong time’.

Glide on the SW face of Magnum, 4-3-19

Mountain Weather

Yesterday: Skies were mostly cloudy with a few patches of sun. There were rain/snow showers on and off throughout the day with 3-6″ of snow falling at upper elevations.   Rain/snowline was around 2000′. Temperatures were in the 20Fs to high 30Fs depending on elevation. Winds were easterly 10-20 mph with gusts into the 40s. Overnight skies were cloudy and easterly wind speeds decreased to 5-10 mph gusting into the teens. Temperatures were in the high 20Fs to high 30Fs.  

Today: Skies will be partly to mostly cloudy. There is a slight chance of rain/snow showers with rain/snowline around 2500′. Temperatures will be in the 40Fs near sea level and range from the high 30Fs to high 20Fs in the upper elevations. Winds will be easterly 5-15 mph with gusts into the 20s. Areas closer to the Arm may see gusts into the 40s again in the afternoon/evening.  

Tomorrow: Continued unsettled weather with rain/snow showers, warm temperatures and the potential for gusty winds. This pattern looks to continue through the week as a series of low-pressure systems move into the Gulf and impact the advisory area.  

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 36   0   0.2   64  
Summit Lake (1400′)  38      0     0   20  
Alyeska Mid (1700′)  35    1     0.45   58  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′)  26 NE    16 48  
Seattle Ridge (2400′)  31  E       8   25